Heat-treatment of cast iron



Jan. 3, 1950 R. R. CAMPBELL- 2,493,339

I HEAT-TREATMENT' OF CAST IRON Filed July" 17, 1946 Z 4- MIN. 5 30 MIN. I EHEAT HEAT TREAT ISOTHERNAL QueNCHaDg/w p v DEGREES muresunmg 0 MINUTES;

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INVENTOR Patented Jan. 3, 1950 UNITED STATES "PATENT OFFICE HEAT-TREATMENT F CAST IRON Robert R. Campbell, Muskegon, Mich.

Application July 1'7, 1946, Serial No. 684,162

6 Claims.

This invention relates to improvements in heat treatment of cast iron.

The principal objects of this invention are:

First, to provide a method for heat treating cast iron which will produce a highly machinable product in a short time.

Second, to provide a method for heat treating ferrous metals requiring a minimum of heating and annealing time.

Third, to provide a method for heat treating cast iron in which the properties and microstructure of the cast iron may be accurately controlled.

Fourth, to provide a method by which cast or malleable iron may be treated to improve its machinability and strength properties at materially reduced cost.

Other objects relating to details and economies of the invention will appear from the description to follow. The invention is defined and pointed out in the claims.

The drawings, of which there is one sheet, illustrate several examples of my heat treating process as applied to diiferent samples of cast iron.

Fig. 1 is a diagrammatic representation of the heat treating process.

Fig. 2 is a chart showing the time and temperature cycles of several heat treatments according to my process.

Cast iron is the term applied to ferrous metals containing from 2.0 to 3.8% carbon and smaller quantities of other materials. At the present time cast iron is prepared for use by pouring in a molten state into molds of the desired shape. These molds may be arranged for quick cooling of the molten metal, in which case the product is known as chilled iron, or for more gradual cooling, in which case the product is known as gray iron, soft iron, or simply cast iron.

In the cooling process the carbon which is soluble in the molten metal separates in the form of iron carbide known as cementite in varying proportions and grain structures depending upon the rate of cooling. Cementite is a very hard and brittle substance which renders the casting unsuitable for machining and general use. To overcome this brittleness it has been the practice to anneal the castings by a process of slowly reheating them to around 1600 or 1750 F. for a considerable period of time, in some cases as high as 40 hours, and to control the cooling of the casting through various temperature ranges for as long as 70 hours.

I have found that the time required to reduce the cementite formation in castings may be greatly reduced by rapidly heating the casting to a higher temperature which is just under the temperature at which the casting will become soft and fluid enough to collapse or lose its proper shape. The casting is held at this high temperature for a short period of time, between three and twenty minutes, and then rapidly cooled to temperatures between 900 and 1500 F. The casting is then held at this lower annealing temperature from between 15 minutes and an hour when it can be air cooled or quenched to room temperature and will have strength and machinability properties which are superior to castings treated in the old manner.

Fig. 1 represents a heat treatment process for a cast cylinder sleeve having a relatively thin wall section.

Reference character I indicates a pre-heat furnace in which the casting is maintained at 1500 F. No time is indicated in this oven since it is merely necessary for the casting to reach the indicated temperature throughout. If desired the casting may be taken at this temperature directly from the casting operation. Reference character 2 indicates the high heat oven which is maintained at 2050 F. and in which the casting is allowed to remain for four minutes. This oven may be heated in any manner, but I have found that a bath of molten salt is well adapted for heating small objects since there is no oxidation of the casting in this form of oven. Reference character 3 indicates an isothermal quench and drawing oven which is maintained at 1250 F. and in which the casting is allowed to remain for 30 minutes. As indicated in the drawings, the quenching oven will be larger than the high heat oven as the casting will remain in it for a longer time and larger capacity is necessary to make full use of the high heat oven.

In the chart shown in Fig. 2 are lines indicating the cycle of various heat treatments. Reference characters 4, 5, and 6 indicate the heat treating cycle of three samples all Of which had a chemical composition including from 3.15 to 3.25% carbon. The cycle indicated at 4 was held at 1000 F., for 10 minutes, raised to 1500 F. for 10 minutes and then raised to 2100 F. for two full minutes. The sample was then quenched to 1100 F. and held for 15 minutes, at which time it was air cooled. The casting had a Brinell hardness number of 269 and was composed of fine pearlite grain structure with not exceeding 5% of primary carbide remaining. Free ferrite did 3 not exceed 20% and the free graphite was fine and uniformly distributed.

The cycle indicated at 5 was held at 1400 F. for 12 minutes and raised immediately to 2100" where it was held for three minutes, then cooled to 1050 F. and held for one-half hour before cooling and testing. This sample had a Brinell hardness of 269 and was composed of very fine lamellar pearlite with no primary carbides remaining and with less than 5% free ferrite remaining. The machinability of the sample was excellent.

In the heat treatment indicated at 6 the sample was held at 1300 F. for 15 minutes and raised to 2000 F. for three minutes and quenched.

to 1200 F. where it was held for one hour'before cooling and testing. This sample had a Brin'ell hardness number of 269 and. was composed of 96% fine lamellar pear-lite with not over' 4% free ferrite and less than 1% primary carbides. The machinability of the sample was good.

In the heat treatment indicated at I the sample was held at 1000 F. for five minutes and raised to 1400 F. for five minutes and then raised to 2050 F. for five minutes. The sample was then air cooled to 1400 F. and held for one hour. The sample had a Brinell hardness number of 197 and a grain structure of fine pearlite and flake graphite with less than free ferrite. No primary carbides remained in the sample. The sample had a carbon content of 2.55% and was what is known as a white malleable casting.

In'the treatment indicated by the dotted line at B, the samples was held at 1300 F. for 10 minutes and then raised to 1950 F. for three minutes and permitted to air cool. This sample had a Brinell hardness number of 302 and showed 10% primary carbides remaining, indicating that the temperature of 1950 F. was insufficient to reduce the primary carbides in three minutes.

In the heat treatment indicated at 9, the sample was held at 1500 F. for 10 minutes and raised to 2150" F. for five minutes. 'When this sample was removed from the furnace it was sufficiently plastic to lose its shape and fall apart, indicating too high a temperature.

I have discovered that 1 primary carbides or cementite in cast or malleable irons may be rapidly reduced and the casting rendered readily machinable by bringing the casting to a uniform temperature of over 1000 F. and then raising it to a temperature of over 1900 F. and just under the temperature atwhich the casting will fail to support its own weight'and shape for a short period of time, and then cooling the casting to between 900 and 1500 F. and maintaining it at this temperature for less than one hour before cooling it to room temperature. Slight variations in the time and temperature factors will be necessary depending on the composition, thickness, and weight of the castings being treated. In these examples given I sought mainly to produce a pearlitic structure of good machinability, as this is the most common structure desired in cast iron.

However, other well known decomposition products of the primary. carbides andaustenite in the cast iron may be obtained by varying the cooling and heat treating temperatures after the high temperature treatment for proportionately short times. These temperatures and times may be accurately controlled to obtain any desired rain structure.

Having thus described my invention, what I claim as new and desire to secure by'Letters Patent is:

temperature of the metal to between 1,900 F. and

2,100 F., holding it at that temperature for between three and thirty minutes, cooling to an annealing temperature between 900 F. and 1500" F., and maintaining it at that annealing temperature between ten and sixty minutes.

3. The method of heat treating raw ferrous cast metals containing primary carbides or free cementite which comprises rapidly raising temperature of the metal to a temperature approximating 2',050 F., holding the metal at such temperature for between three and twenty minutes, rapidly reducing the temperature of the metal from such high temperature to between 900 F. and 1500 F., and holding it at that temperature for between ten and sixty minutes.

4. The method of heat treating raw ferrous metal castings containing primary carbides or free cementite which comprises rapidly raising the temperature of the metal to a temperature approximating 2000 F., holding the metal at that temperature from three to twenty minutes, subjectin'g the metal to rapid cooling from such high temperature to between 900 F. and 1500 F., and holding it at that temperature for not over one hour.

5. The method of heat treating raw ferrous metal castings containing primary carbides or free cementite which comprises rapidly raising the temperature of the metal toa temperature of approximately 2,100 F., holding it at that temperature for approximately ten minutes, cooling to approximately 1,300" F. and holding it at that temperature for approximately forty minutes.

6. The method of heat treating raw ferrous cast metals containing primary carbides or free cementite which comprises rapidly raising the temperature of the metal to a temperature approximating 1,950 F., holding the metal to such temperature for between six and thirty minutes, subjecting the metal to'rapid cooling to between 900 F. and 1500 F., and holding it at that cooling temperature for" between ten and sixty minutes.

ROBERT R. CAMPBELL.

REFERENCES CITED "The following references are of record in the file of this patent:

UNITED STATES PATENTS Transactions, American Foundrymens Association, published by theAssociation, Chicago, Ill., Vol.48, pp. 283-302; vol. 40, pp. 88-124; vol. 49, p. 452. 

1. THE METHOD OF HEAT TREATING RAW FERROUS METAL CASTINGS CONTAINING PRIMARY CARBIDES OR FREE CEMENTILE WHICH COMPRISES RAPIDLY RAISING THE CASTING HAVING A TEMPERATURE OF APPROXIMATELY 1,000*F. TO A TEMPERATURE BETWEEN 1,900*F. AND 2,100*F., HOLDING IT AT THAT TEMPERATURE BETWEEN TWO AND TWENTY MINUTES, RAPIDLY COOLING THE METAL TO A TEMPERATURE BETWEEN 1,000*F. TO 1,400*F., AND HOLDING IT AT THAT TEMPERATURE BETWEEN TEN AND SIXTY MINUTES. 